What is an Agent Harness?

Harness engineering is how we build systems around models to turn them into work engines. A harness is every piece of code, configuration, and execution logic that isn't the model itself. A raw model is not an agent, but can become one when a harness gives it things like state, tool execution, feedback loops, and enforceable constraints. Concretely, a harness includes things like: • System Prompts • Tools, Skills, MCPs + and their descriptions • Bundled Infrastructure (filesystem, sandbox, browser) • Orchestration Logic (subagent spawning, handoffs, model routing) • Hooks/Middleware for deter